Large numerical aperture objective lens and night vision optical device using the same

ABSTRACT

A large numerical aperture objective lens is disclosed having three lens groups. In order from the object side, these lens groups include: a first lens group of positive refractive power that is formed of two positive lens elements and an achromatic set of paired lens elements formed of a lens element of negative refractive power and a lens element of positive refractive power, respectively; a second lens group of negative refractive power; and a third lens group of positive refractive power, with the surface of the third lens group nearest the object side being convex. Specified conditions are satisfied in order to provide a compact, large numerical aperture objective lens having its aberrations favorably corrected as well as to correct for shifts of focus caused by expansion/contraction of the lens barrel with changes in temperature. The objective lens of the invention is particularly suitable for use in a night vision optical device having a photoelectron amplifier tube and an eyepiece, wherein aberrations in the eyepiece are balanced by aberrations of the objective lens.

BACKGROUND OF THE INVENTION

[0001] A lens is described in Japanese Examined Patent Application(Kokoku) H7-95143 that is well-known as a large numerical apertureobjective lens. This lens has an F-number of about 1.85 with a visualfield of about 12°, and thus provides a bright telescopic lens whichfavorably corrects distortion and other aberrations. This lens is wellsuited for use as an objective lens for a photographic camera or a videocamera. However, an objective lens for a night vision optical devicethat uses a photoelectron amplifier tube must have an even smallerF-number in order to provide a sufficiently bright image. Moreover, anobjective lens for a night vision optical device must generally producea large negative distortion. The reason is that the eyepiece in a nightvision optical device generally produces a positive distortionaberration, and correction of such distortion in the eyepiece alone isvery difficult. Therefore, a technique is generally used wherein a largenegative distortion of about −4% to about −9% is generated in theobjective lens for a night vision optical device in order to cancel thepositive distortion of the eyepiece. In this manner, favorablecorrection of distortion of the night vision optical device is provided.

[0002] Because the objective lens of a night vision optical device doesnot require a large back focus, and a compact objective lens is desired,a telescopic lens would normally be suitable, except for the requirementthat the objective lens produce a large negative distortion, asdescribed above. Although it would appear that an objective lens havinga large negative distortion could be readily designed by simplyarranging a surface having a strongly negative refractive power near theobject side of a lens system or near the image side of a lens system,such an arrangement makes it difficult, in a compact arrangement of lenselements, to favorably correct both coma and astigmatism.

BRIEF SUMMARY OF THE INVENTION

[0003] A first object of the present invention is to provide a compact,large numerical aperture objective lens which provides a bright imageand which produces a large negative distortion aberration. A secondobject of the present invention is to provide a night vision opticaldevice which uses such an objective lens.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

[0005]FIG. 1 shows the basic lens element configuration of a largenumerical aperture objective lens according to Embodiment 1 of thepresent invention;

[0006]FIG. 2 shows the basic lens element configuration of a nightvision optical device that includes the large numerical apertureobjective lens shown in FIG. 1;

[0007] FIGS. 3A-3D show various aberrations of the large numericalaperture objective lens of Embodiment 1 of the present invention;

[0008] FIGS. 4A-4D show various aberrations of the large numericalaperture objective lens of Embodiment 2 of the present invention;

[0009] FIGS. 5A-5D show various aberrations of the large numericalaperture objective lens of Embodiment 3 of the present invention; and

[0010] FIGS. 6A-6D show various aberrations of the large numericalaperture objective lens of Embodiment 4 of the present invention.

DETAILED DESCRIPTION

[0011] This invention relates to a large numerical aperture objectivelens and, more particularly, to a large numerical aperture objectivelens suitable for a night vision optical device that uses aphotoelectron amplifier tube.

[0012] The large numerical aperture objective lens includes, in orderfrom the object side: a first lens group having positive refractivepower, a second lens group having negative refractive power, and a thirdlens group having positive refractive power. The first lens group isformed of, in order from the object side, two positive lens elements anda set of paired lens elements. The set of paired lens elements isachromatic and is formed of a positive lens element and a negative lenselement that are either joined or separated by an air space. The lenssurface of the third lens group that is nearest the object side isconvex, and the following Conditions (1)-(4) are satisfied:

0.45<|f_(G1)|/f<0.9  Condition (1)

0.3<|f_(G2)|/f<0.7  Condition (2)

0.15<|R_(G31)|/f<0.6  Condition (3)

0.4<D_(G2G3)/f<0.8  Condition (4)

[0013] where

[0014] f is the focal length of the objective lens,

[0015] f_(G1) is the focal length of the first lens group,

[0016] f_(G2) is the focal length of the second lens group,

[0017] R_(G31) is the radius of curvature of the surface of the thirdlens group nearest the object side, and

[0018] D_(G2G3) is the on-axis distance between the second lens groupand the third lens group.

[0019] Moreover, it is desirable that the third lens group include a setof paired lens elements having positive refractive power overall, withthe set of paired lens elements being achromatic and formed of apositive lens element and a negative lens element.

[0020] Furthermore, it is preferable that, of among the lens elements inthe first and second lens groups, that at least one lens element hasnegative refractive power and is made of a material that satisfies thefollowing Condition (5):

dn/dT>7×10⁻⁶  Condition (5)

[0021] where

[0022] dn/dT is the temperature coefficient of the index of refractionof the material at room temperature.

[0023] Also, it is preferable that at least one lens element having apositive refractive power in the first lens group is made of a materialsatisfying the following Conditions (6) and (7):

dn/dT<0  Condition (6)

α>9×10⁻⁶  Condition (7)

[0024] where

[0025] dn/dT is the temperature coefficient of the index of refractionof the material at room temperature, and

[0026] α is the linear expansion coefficient of the material at roomtemperature.

[0027] The night vision optical device of the present invention includesthe above-described large numerical aperture objective lens, aphotoelectron amplifier tube and an eyepiece.

[0028] Moreover, the above “paired lenses” means a cemented lens, or twolenses which are arranged with an air space separating them and whichhave two surfaces of roughly equal curvature that face one another.

[0029]FIG. 1 is representative of the basic lens element configurationof each embodiment of the large numerical aperture objective lens of theinvention. This large numerical aperture objective lens includes, inorder from the object side, a first lens group G₁ having positiverefractive power, a second lens group G₂ having negative refractivepower and a third lens group G₃ having positive refractive power. Thefirst lens group G₁ includes, in order from the object side, twopositive lens elements L₁ and L₂, and an achromatic set of paired lenselements L₃ and L₄, formed of a negative and a positive lens element,respectively. The second lens group G₂ is formed of a negative lenselement L₅. The third lens group G₃ is formed of an achromatic set ofpaired lens elements having a surface nearest the object side that isconvex. The achromatic set of paired lens elements is formed of apositive lens element L₆ and a negative lens element L₇ which may becemented together.

[0030] A light beam from an object is converged by these seven lenselements L₁-L₇ and is imaged onto a surface located at position P.Moreover, the position P in the figure is located at the surface of aface plate (i.e., a cover glass) 1 that forms the plane of incidence ofthe photo-electron amplifier tube.

[0031] This large numerical aperture objective lens of the presentinvention satisfies the above Conditions (1)-(4).

[0032]FIG. 2 shows the basic lens element configuration of a nightvision optical device that includes the large numerical apertureobjective lens shown in FIG. 1. In FIG. 2, the lens barrel 5 is shownusing chain lines with two dots in each link. The night vision opticaldevice includes a large numerical aperture objective lens 10, aphotoelectron amplifier tube 2, and an eyepiece 4, which are held inplace within the lens barrel 5. A light beam from an object is convergedby the large numerical aperture objective lens 10 and is imaged onto therear surface of a face plate 1 located at point P. The rear surface offace plate 1 is the input surface to the photoelectron amplifier tube 2.Thus, the image intensity is enhanced by the action of the photoelectronamplifier tube 2, and may be observed from the eye position E.P. via theeyepiece 4 by an observer as an image having increased brightness.

[0033] The purposes of Conditions (1)-(7) will now be described.

[0034] Condition (1) specifies the focal length of the first lens groupG₁ relative to the focal length of the objective lens. If the lowerlimit of Condition (1) is not satisfied, spherical aberration, coma andastigmatism generated by the first lens group become excessive, and itbecomes difficult to favorably correct these aberrations. If the upperlimit of Condition (1) is not satisfied, compactness of the objectivelens will be difficult to obtain.

[0035] Condition (2) specifies the focal length of the second lens groupG₂ relative to the focal length of the objective lens. If the lowerlimit of Condition (2) is not satisfied, spherical aberration, coma andastigmatism generated by the second lens group become excessive, and itbecomes difficult to favorably correct these aberrations. If the upperlimit of Condition (2) is not satisfied, compactness of the objectivelens will be difficult to obtain.

[0036] Condition (3) specifies the radius of curvature of the surface ofthe third lens group nearest the object side. As this surface is convex,the function of this surface is to generate a large negative distortion.If the lower limit of Condition (3) is not satisfied, it becomesdifficult to correct the coma and image surface curvature. If the upperlimit of Condition (3) is not satisfied, it becomes difficult togenerate sufficient negative distortion as needed by an objective lensfor a night vision optical device.

[0037] Condition (4) specifies the on-axis spacing between the secondlens group G₂ and the third lens group G₃. If the lower limit ofCondition (4) is not satisfied, it becomes difficult to favorablycorrect the spherical aberration. If the upper limit of Condition (4) isnot satisfied, compactness of the objective lens will be difficult toobtain.

[0038] Moreover, a light beam can be slowly converged in order tosuppress the occurrence of high-order aberrations by forming the firstlens group G₆, in order from the object side, of two positive lenselements L₁ and L₂, as well as of cemented lens elements L₃, L₄. Theaxial chromatic aberration can be favorably corrected by the cementedlens elements L₃, L₄ being formed of a negative lens element L₃ and apositive lens element L₄. Furthermore, the lateral color can befavorably corrected by the third lens group G₃ being a cemented lensformed of a positive lens element L₆ and a negative lens element L₇.Further, instead of using a cemented lens in each of the first lensgroup G₁ and the third lens group G₃, each cemented lens can instead beformed as two lens elements having roughly equal curvature and separatedby an air space. Hereinafter, such lenses and cemented lenses will betermed “paired lenses”. These paired lenses may also be arranged, inorder from the object side, using a negative lens element and a positivelens element.

[0039] Thus, the present invention enables the objective lens togenerate a large negative distortion in order to compensate for thelarge positive distortion generated by the eyepiece 4 for the nightvision optical device, as well as to correct other aberrations andobtain a bright image using a compact objective lens having a largenumerical aperture.

[0040] Moreover, as described below, it is desired that the lenselements of the large numerical aperture objective lens relating to thepresent invention are made of a material satisfying Conditions (5)-(7).However, as illustrated by some embodiments of the present invention, itis not necessary to satisfy all of the Conditions (5)-(7). For example,as in Embodiment 1, it is acceptable to satisfy Conditions (6) and (7)but not Condition (5). It is also acceptable to satisfy only Condition(5) and not Conditions (6) and (7).

[0041] This large numerical aperture objective lens becomes a lenssuited to a night vision optical device by using lens elements that aremade of material(s) satisfying the above Conditions (5)-(7). Generally,once an objective lens is incorporated into a night vision opticaldevice, it is difficult to make positional adjustments, such asfocusing. Therefore, a shift of focusing position due to temperaturechange becomes a problem in the case of using the night vision opticaldevice under a temperature greatly different from normal temperature.Such a shift of focusing position occurs due to the index of refractionof the lens material changing with temperature, the linearexpansion/contraction of the lens materials with increased/decreasedtemperature, and linear expansion/contraction of the said lens barrel 5with increased/decreased temperature. In particular, aluminum isgenerally used as the construction material of the lens barrel due tosuch reasons as cost, weight, ease of manufacture and so on. However,the linear expansion coefficient of aluminum at the normal designtemperature is as large as 23.1×10⁻⁶, which thus causes a shifting inthe focus position due to the lens barrel changing length withtemperature change from the design temperature of the night visionoptical device.

[0042] However, the above Conditions (5)-(7) specify performancecriteria which the material(s) of the objective lens elements mustsatisfy, and which enable focus shifts due to temperature changes to besuppressed. If these conditions are not satisfied, compensation for theexpansion/contraction of the lens barrel 5 due to temperature changesbecomes difficult, and the focus position shifts to the object side whenthe temperature rises, and to the image side when the temperature falls.By satisfying Conditions (5), (6) and (7), the focusing of the objectivelens shifts to the image side when the temperature rises and to theobject side when the temperature falls. Thus, correction of the focusingposition is obtained which compensates for the lens barrel expandingwhen the temperature rises, as well as for the lens barrel contractingwhen the temperature falls.

[0043] Specific embodiments of the present invention are describedbelow.

Embodiment 1

[0044]FIG. 1 shows the basic lens element configuration of Embodiment 1.The first lens group G₁ is formed of, in order from the object side, afirst lens element L₁ that is a positive meniscus lens with its convexsurface on the object side, a second lens element L₂ that is a positivemeniscus lens with its convex surface on the object side, and a cementedlens that is formed of a negative meniscus lens element L₃ with itsconcave surface on the image side cemented to a positive meniscus lenselement with its convex surface on the object side. The second lensgroup G₂ is formed of a fifth lens element L₅ that is a negativemeniscus lens with its convex surface on the object side. The third lensgroup G₃ is a cemented lens formed of lens elements L₆, L₇. The sixthlens element L₆ is biconvex with surfaces of different radii ofcurvature, and with the surface of smaller radius of curvature on theobject side. The seventh lens element L₇ is biconcave with surfaceshaving different radii of curvature, and with the surface of smallerradius of curvature on the image side.

[0045] Table 1 below lists the surface number # in order from the objectside, the radius of curvature R (in mm) of each surface, the on-axisspacing D (in mm) between surfaces, as well as the index of refractionN_(d) and the Abbe number υ_(d) (both at the d line) of each lenselement of the objective lens of Embodiment 1. Moreover, valuescorresponding to the above Conditions (1)-(4) for Embodiment 1 are shownin the middle section of Table 1. Values relating to the aboveConditions (6) and (7) and the focusing position shift ΔBF when thelarge numerical aperture objective lens of this embodiment is mounted toan aluminum lens barrel and the temperature rises from 20° C. to 40° C.are shown in the lower section of Table 1. The nearer the values ΔBF areto zero, the smaller the shift of focusing position due to temperaturechange. TABLE 1 # R D N_(d) υ_(d) 1 119.310 6.57 1.618 63.4 2 485.6200.65 3 52.817 12.13 1.618 63.4 4 156.400 0.65 5 53.877 3.52 1.805 25.5 632.007 11.67 1.697 55.5 7 61.614 6.78 8 243.470 2.41 1.593 35.5 9 29.35665.60 10 20.254 10.85 1.773 49.6 11 −40.175 2.26 1.689 31.2 12 24.8726.15 13 ∞ 5.09 1.498 65.1 14 ∞ f = 100.0 (mm) |f_(G1)|/f = 0.6856|f_(G2)|/f = 0.5655 |R_(G31)|/f = 0.2026 D_(G2G3)/f = 0.6562 RefractivePower dn/dT α First lens element L₁ positive −2.6 × 10⁻⁶ 9.2 × 10⁻⁶Second lens element L₂ positive −2.6 × 10⁻⁶ 9.2 × 10⁻⁶ Focusing positionshift: ΔBF = −0.022 mm

[0046] As is evident from Table 1, Embodiment 1 satisfies the aboveConditions (1)-(4), (6) and (7).

[0047] FIGS. 3A-3D show the spherical aberration, astigmatism,distortion and lateral color, respectively, for Embodiment 1.

Embodiment 2

[0048] The large numerical aperture objective lens of Embodiment 2 hasthe same basic lens element configuration as that of Embodiment 1.

[0049] Table 2 below lists the surface number # in order from the objectside, the radius of curvature R (in mm) of each surface, the on-axisspacing D (in mm) between surfaces, as well as the index of refractionN_(d) and the Abbe number υ_(d) (both at the d line) of each lenselement of the objective lens of Embodiment 2. Values corresponding tothe above Conditions (1)-(4) for Embodiment 2 are shown in the middlesection of Table 2. Values relating to the above Condition (5) and thefocusing position shift ΔBF when the large numerical aperture objectivelens of this embodiment is mounted to an aluminum lens barrel and thetemperature rises from 20° C. to 40° C. are shown in the lower sectionof Table 2. The nearer the values ΔBF are to zero, the smaller the shiftof focusing position due to temperature change. TABLE 2 # R D N_(d)υ_(d) 1 94.991 7.59 1.603 60.6 2 540.470 0.56 3 48.226 10.37 1.603 60.64 122.620 0.65 5 48.351 3.24 1.805 25.4 6 29.314 11.19 1.658 50.9 760.772 11.18 8 175.090 2.03 1.805 25.4 9 26.165 50.07 10 19.786 7.781.786 44.2 11 −136.360 3.81 1.648 33.8 12 26.236 6.95 13 ∞ 5.05 1.48770.4 14 ∞ f = 100.0 (mm) |f_(G1)|/f = 0.6130 |f_(G2)|/f = 0.3844|R_(G31)|/f = 0.1978 D_(G2G3)/f = 0.5006 Refractive Power dn/dT Thirdlens element L₃ negative 10.1 × 10⁻⁶ Fifth lens element L₅ negative 10.1× 10⁻⁶ Focusing position shift: ΔBF = −0.039 mm

[0050] As is evident from Table 2, Embodiment 2 satisfies the aboveConditions (1)-(5).

[0051] FIGS. 4A-4D show the spherical aberration, astigmatism,distortion and lateral color, respectively, for Embodiment 2.

Embodiment 3

[0052] The large numerical aperture objective lens of Embodiment 3 hasthe same basic lens element configuration as that of Embodiment 1.

[0053] Table 3 below lists the surface number # in order from the objectside, the radius of curvature R (in mm) of each surface, the on-axisspacing D (in mm) between surfaces, as well as the index of refractionN_(d) and the Abbe number υ_(d) (both at the d line) of each lenselement of the objective lens of Embodiment 3. Moreover, valuescorresponding to the above Conditions (1)-(4) for Embodiment 3 are shownin the middle section of Table 3. Values relating to the above Condition(5) and the focusing position shift ΔBF when the large numericalaperture objective lens of this embodiment is mounted to an aluminumlens barrel and the temperature rises from 20° C. to 40° C. are shown inthe lower section of Table 3. The nearer the values ΔBF are to zero, thesmaller the shift of focusing position due to temperature change. TABLE3 # R D N_(d) υ_(d) 1 54.632 10.65 1.517 64.2 2 201.860 0.46 3 51.6859.65 1.517 64.2 4 165.130 0.46 5 38.682 3.24 1.785 25.7 6 28.037 10.651.623 56.9 7 53.545 4.02 8 262.460 2.78 1.762 26.6 9 24.065 53.69 1020.144 8.12 1.720 50.3 11 −74.321 3.24 1.648 33.8 12 30.127 2.31 13 ∞5.05 1.487 70.4 14 ∞ f = 99.2 (mm) |f_(G1)|/f = 0.5276 |f_(G2)|/f =0.3532 |R_(G31)|/f = 0.2030 D_(G2G3)/f = 0.5411 Refractive Power dn/dTThird lens element L₃ negative 7.7 × 10⁻⁶ Fifth lens element L₅ negative7.3 × 10⁻⁶ Focusing position shift: ΔBF = −0.032 mm

[0054] As is evident from Table 3, Embodiment 3 satisfies the aboveConditions (1)-(5).

[0055] FIGS. 5A-5D show the spherical aberration, astigmatism,distortion and lateral color, respectively, for Embodiment 3.

Embodiment 4

[0056] The large numerical aperture objective lens of Embodiment 4 hasthe same basic lens element configuration as that of Embodiment 1.

[0057] Table 4 below lists the surface number # in order from the objectside, the radius of curvature R (in mm) of each surface, the on-axisspacing D (in mm) between surfaces, as well as the index of refractionN_(d) and the Abbe number υ_(d) (both at the d line) of each lenselement of the objective lens of Embodiment 4. Moreover, valuescorresponding to the above Conditions (1)-(4) for Embodiment 4 are shownin the middle section of Table 4. Values relating to the aboveConditions (5)-(7) and the focusing position shift ΔBF when the largenumerical aperture objective lens of this embodiment is mounted to analuminum lens barrel and the temperature rises from 20° C. to 40° C. areshown in the lower section of Table 4. The nearer the values ΔBF are tozero, the smaller the shift of focusing position due to temperaturechange. TABLE 4 # R D N_(d) υ_(d) 1 92.362 7.59 1.618 63.3 2 358.6100.93 3 50.240 10.37 1.618 63.3 4 144.750 0.28 5 47.014 3.24 1.805 25.4 629.706 11.19 1.667 48.3 7 55.105 11.18 8 175.090 2.03 1.805 25.4 926.165 50.07 10 19.786 7.78 1.786 44.2 11 −136.360 3.81 1.648 33.8 1226.236 6.95 13 ∞ 5.05 1.487 70.4 14 ∞ f = 100.1 (mm) |f_(G1)|/f = 0.6116|f_(G2)|/f = 0.3842 |R_(G31)|/f = 0.1977 D_(G2G3)/f = 0.5003 RefractivePower dn/dT α First lens element L₁ positive −3.5 × 10⁻⁶ 10.1 × 10⁻⁶Second lens element L₂ positive −3.5 × 10⁻⁶ 10.1 × 10⁻⁶ Third lenselement L₃ negative 10.1 × 10⁻⁶ Fifth lens element L₅ negative 10.1 ×10⁻⁶ Focusing position shift: ΔBF = −0.001 mm

[0058] As is evident from Table 4, Embodiment 4 satisfies the aboveConditions (1)-(7).

[0059] FIGS. 6A-6D show the spherical aberration, astigmatism,distortion and lateral color, respectively, for Embodiment 4.

[0060] As shown in the various aberration curves for each embodiment,the objective lenses of each embodiment have F numbers as low as1.41-1.52 and a large negative distortion while suppressing theoccurrence of other aberrations.

[0061] The invention being thus described, it will be obvious that thesame may be varied in many ways. For example, the radii of curvature Rand on-axis surface spacings D can be readily scaled to achieve a lenshaving a different focal length. Furthermore, although the largenumerical aperture objective lens of the present invention isparticularly useful when it is used in a night vision optical device,the use of the large numerical aperture objective lens in the inventionis not restricted to night vision applications, as it can be useful inother applications as well. And, although the night vision opticaldevice illustrated herein is of the monocular type, the night visionoptical device can instead be made binocular by adding a secondobjective lens. Thus, such variations are not to be regarded as adeparture from the spirit and scope of the invention. Rather the scopeof the invention shall be defined as set forth in the following claimsand their legal equivalents. All such modifications as would be obviousto one skilled in the art are intended to be included within the scopeof the following claims.

What is claimed is:
 1. An objective lens comprising, in order from theobject side: a first lens group having positive refractive power andformed of, in order from the object side, two positive lens elements andan achromatic set of paired lens elements formed of a negative lenselement and a positive lens element; a second lens group having negativerefractive power; and a third lens group having positive refractivepower, the surface of the third lens group nearest the object side beingconvex; wherein the following Conditions (1)-(4) are satisfied0.45<|f_(G1)|/f<0.9  Condition (1)0.3<|f_(G2)|/f<0.7  Condition(2)0.15<|R_(G31)|/f<0.6  Condition (3)0.4<D_(G2G3)/f<0.8  Condition (4)where f is the focal length of the objective lens, f_(G1) is the focallength of the first lens group, f_(G2) is the focal length of the secondlens group, R_(G31) is the radius of curvature of the surface of thethird lens group nearest the object side, and D_(G2G3) is the on-axisdistance between the second lens group and the third lens group.
 2. Theobjective lens of claim 1, said third lens group being formed of anachromatic set of paired lenses having overall positive refractive powerthat is formed of a positive lens element and a negative lens element.3. The objective lens of claim 1, wherein at least one lens element, ofamong the lens elements having negative refractive power in the firstand second lens groups, is made of a material satisfying the followingCondition (5): dn/dt>7×10⁻⁶  Condition (5) where dn/dT is thetemperature coefficient of the index of refraction of the material atroom temperature.
 4. The objective lens of claim 2, wherein at least onelens element, of among the lens elements having negative refractivepower in the first and second lens groups, is made of a materialsatisfying the following Condition (5): dn/dT>7×10⁻⁶  Condition (5)where dn/dT is the temperature coefficient of the index of refraction ofthe material at room temperature.
 5. The objective lens of claim 1,wherein at least one lens element in said first lens group is made of amaterial satisfying the following Conditions (6) and (7):dn/dt<0  Condition (6)α>9×10⁻⁶  Condition (7) where dn/dT is thetemperature coefficient of the index of refraction of the material atroom temperature, and α is the linear expansion coefficient of thematerial at room temperature.
 6. The objective lens of claim 2, whereinat least one lens element in said first lens group is made of a materialsatisfying the following Conditions (6) and (7): dn/dT<0  Condition(6)α>9×10⁻⁶  Condition (7) where dn/dT is the temperature coefficient ofthe index of refraction of the material at room temperature, and α isthe linear expansion coefficient of the material at room temperature. 7.The objective lens of claim 3, wherein at least one lens element in saidfirst lens group is made of a material satisfying the followingConditions (6) and (7): dn/dT<0  Condition (6)α>9×10⁻⁶  Condition (7)where dn/dT is the temperature coefficient of the index of refraction ofthe material at room temperature, and α is the linear expansioncoefficient of the material at room temperature.
 8. A night visionoptical device, comprising the objective lens of claim 1 in combinationwith a photoelectron amplifier tube and an eyepiece.
 9. A night visionoptical device, comprising the objective lens of claim 2 in combinationwith a photoelectron amplifier tube and an eyepiece.
 10. A night visionoptical device, comprising the objective lens of claim 3 in combinationwith a photoelectron amplifier tube and an eyepiece.
 11. A night visionoptical device, comprising the objective lens of claim 5 in combinationwith a photoelectron amplifier tube and an eyepiece.